1. Field of the Invention
The invention relates generally to the field of mass analysis. The invention more specifically relates to a method and apparatus for gas-phase analysis of organic compounds at low concentrations in test samples.
2. Description of the Prior Art
As is generally well known, problems associated with mass analyzers limit the range of concentrations over which organic compounds can be detected and analyzed in the gas phase. Test samples usually must be concentrated in an enrichment step prior to analysis. Because complicated procedures for taking the sample and concentrating it cannot be standardized, considerable deviation and error in measurement occur. Considerable amounts of the test sample are lost by the use of gas sampling devices such as gas syringes for transfer of the concentrated sample to the analyzer. Additionally, gas phase reactions continue during transfer of the sample to the analyzer, further impairing the analysis. Very rarely is the detector satisfactorily combined with the sampling or reaction volume, and in such cases the systems are based on special spectroscopic methods.
Conventional mass analyzers cannot be used for the direct detection and measurement of organic compounds in ppb concentrations. The low signal-to-noise ratio at regular pressures of 10.sup.-4 to 10.sup.-6 torr prevents analysis in the ppb range. A straight increase in the vacuum reduces the concentration of the chemicals below the detection limit. These conventional mass analyzers include single-stage magnet sector units, and more recently introduced single-stage quadrupole units.
No practical device for directly analyzing chemicals in the gas phase in ppb concentrations was previously available which operated without a preliminary enrichment (concentration) step. For a mass analyzer using a single-stage magnet sector to obtain the required resolution and sensitivity, a very large magnet is required, resulting in a very massive machine. An alternative approach is to use two or more stages of magnet sectors or quadrupole units in which the first stage, in effect, provides a preliminary enrichment or concentration for the second step. Such multiple stage machines are more complicated and still tend to be physically large. Their relatively large size and high cost generally preclude their use for on-site sampling or the continuous monitoring of industrial processes.